1. Field of the Invention
The present invention relates to a glass composition for a thick-film resistor paste suitable for use for forming a thick-film resistor having a high resistance, a thick-film resistor paste using this glass composition, a thick-film resistor, and an electronic device.
2. Description of the Related Art
A resistor paste is generally mainly comprised of a glass composition, a conductive material, and an organic vehicle. The glass composition is included for adjusting the resistance value and having adhesion. The resistor paste is printed on a substrate, then fired to form a thick-film (5 to 20 μm) resistor. This type of resistor paste (thick-film resistor) usually contains ruthenium oxide (RuO2) or a lead-ruthenium oxide as a conductive material and lead oxide (PbO)-based glass as the glass material and therefore is a leaded paste.
In recent years, environmental issues have become hotly debated. Efforts are being made to eliminate lead and other harmful substances from electronic devices. Resistor pastes and thick-film resistors are no exception. Research is now being conducted to make them lead free.
One of the issues in making resistor paste lead-free is achieving both a good temperature characteristic (TCR) and short time overload characteristic (STOL) in a resistor paste with a high resistance (1 kΩ/□ or more). For example, if applying the practice of adjustment of the TCR by addition of a metal oxide as used in conventional lead-based resistor pastes as it is to lead-free compositions, the fluctuation in the resistance value due to application of voltage will become greater than with lead-based compositions, so realizing both good TCR and STOL will be difficult.
In view of this situation, attempts have been made to achieve both a good temperature characteristic (TCR) and short time overload characteristic (STOL) by a resistor paste mainly comprised of a lead-free glass composition, lead-free conductive material, and organic vehicle containing CaTiO3 or NiO as an additive.
For example, Japanese Patent Publication (A) No. 2003-197405 describes that it is preferable to introduce into a resistor paste for example CaTiO3 in an amount of over 0 vol % to not more than 13 vol % or NiO in an amount of over 0 vol % to not more than 12 vol %, more preferable to simultaneously add CuO, ZnO, MgO, and other additives. Japanese Patent Publication (A) No. 2003-197405 describes that by simultaneously adding these additives, it is possible to provide a lead-free resistor paste suitable for obtaining a resistor having a high resistance value, but having a small temperature characteristic of the resistance value (TCR) and short time overload characteristic (STOL).
However, in a resistor formed using a resistor paste adjusted in TCR by introducing a large amount of additives like in the invention described in Japanese Patent Publication (A) No. 2003-197405, the STOL tends to fall more compared with the case of using a resistor paste of a conventional lead-based composition. Therefore, it is believed that if it were possible to further improve the STOL characteristic in a resistor formed using a resistor paste of a composition with no addition of any additives, that is, comprised of just a glass composition and conductive material, it would be possible to avoid the drop in the STOL characteristic.
Further, Japanese Patent Publication (A) No. 2003-197405 does indeed show improvements in the TCR and STOL and also discloses samples with a TCR within ±100 ppm and a STOL close to zero. However, sufficiently good values of both TCR and STOL are only obtained in extremely limited compositions. In the majority of compositions, the STOL is a value of 1% or more even if small. If compositions giving sufficiently good values of both TCR and STOL are limited in this way, for example, the freedom of other characteristics is also limited and problems are liable to occur in the design of the resistor paste, so more improvement is desired.